The invention relates to measuring instruments, and more particularly to a non-contact inductive pickup for determining the interface between two media.
Existing non-contact inductive pickups for determining the interface between two media having different specific density and different specific resistance depend for their operation on the distortion of their electromagnetic fields by the fields produced by eddy currents which appear at least in one of the media, which is electrically conductive and non-magnetic.
A known existing non-contact inductive pickup for determining the interface between two media comprises a non-contact inductive proximity pickup wherein two excitation windings are disposed on the end legs of an E-shaped magnetic circuit and are so connected that the magnetic flux produced in the central leg on which the measuring winding is disposed is equal to the difference between the magnetic fluxes produced by the excitation windings. When the excitation windings produce equal fluxes, the magnetic flux in the central leg is zero. The magnetic circuit of the pickup is disposed so as to be parallel to the interface between two media. The container holding the two media, the interface between which is to be determined, is disposed between one of the end legs and the central leg of the magnetic circuit. When the interface between the two media is located between this end leg and the central leg, the stable equilibrium of the magnetic fluxes in the central leg is disturbed and an emf is induced in the measuring winding.
Such pickups determine the interface between air and liquid metal when set up near the walls of a vessel or ladle. However these pickups cennot be used for determining the interface between two media in electrometallurgical processes such as electroslag refining, plasma-arc refining, electron-beam refining, electroslag welding, since they operate only if the container holding the two media is made from a non-magnetic material, however ingot molds wherein the interface between liquid metal and slag or between liquid metal and gaseous medium has to be determined, are made from copper. Hence such pickups have to be made excessively large and additional calculations and experiments are necessary for each specific case.
A non-contact inductive pickup is also known for determining the interface between two media using a single open E-shaped magnetic circuit. It comprises two operating windings disposed on the end legs of the magnetic circuit and connected so that the magnetic fluxes set up by the current through these windings in the central leg carrying the measuring winding are mutually subtracted. As long as the magnetic fluxes are equal, the emf in the measuring winding is zero. If an electrically conductive medium is placed between one of the end legs and the central leg the eddy currents which are induced in the medium correspondingly reduce one of the magnetic fluxes. As a result the magnetic fluxes affecting the central leg will no longer be equal and an emf will be induced in the measuring winding. The non-contact inductive pickup for determining the interface between two media which uses an E-shaped magnetic circuit has a number of serious drawbacks, one of them being the small depth to which the electromagnetic field penetrates into the media, the interface between which is to be determined.
The penetration depth and the pattern of the magnetic lines of force depend on the spacing between the end legs and the central leg of the E-shaped magnetic circuit: the greater the spacing, the deeper the penetration of the electromagnetic field.
Hence a pickup of this type is difficult to use if the distance from the pickup to the interface between two media which is to be determined is 10-15 mm as, in this case, the spacing between the central and end legs of the magnetic circuit has to be increased, which results the pickup as a whole having much larger dimensions.
The disadvantages mentioned above restrict the application of such a pickup.